1. Field of the Invention
The present invention relates to a variable demultiplexer, and particularly to a variable demultiplexer using optical Bloch oscillations.
2. Description of the Background Art
In recent years, demands for large-capacity optical communication networks have been remarkably increasing. As one of techniques for meeting such demands, there has been wavelength division multiplex transmission. For using this technique, changes have occurred in main devices, and it is now required to use optical control devices such as a variable demultiplexer. Variable demultiplexers using optical Bloch oscillations or photonic crystals have been known as the above kind of variable demultiplexers.
The variable demultiplexer using the optical Bloch oscillations is disclosed, e.g., in “OPTICS LETTERS”, (U.S.A.), Optical Society of America, Nov. 1, 1998, Vol. 23, No. 21, pp. 1701-1703 and “PHYSICAL REVIEW LETTERS” (U.S.A.), The American Physical Society, Dec. 6, 1999, Vol. 83, No. 23, pp. 4756-4759. FIG. 24 is a schematic perspective view of the variable demultiplexer disclosed in “OPTICS LETTERS”, (U.S.A.), Optical Society of America, Nov. 1, 1998, Vol. 23, No. 21, pp. 1701-1703. FIG. 25 is a graph illustrating a relationship between a position in an X-axis direction and a refractive index gradient in the variable demultiplexer shown in FIG. 24. FIG. 26 is a schematic plan of the variable demultiplexer disclosed in the above “PHYSICAL REVIEW LETTERS”. Referring to FIGS. 24-26, these conventional variable demultiplexers will now be described.
As shown in FIG. 24, the variable demultiplexer disclosed in the above “OPTICS LETTERS” has a waveguide array formed of waveguides 122 which are cyclically arranged on a substrate 121 with spaces therebetween. The upper side of the waveguide array is covered with an electrode 123. A terminal 126b is connected to electrode 123. A terminal 126a is connected to substrate 121. The spaces between waveguides 122 are constant. Each waveguide 122 has the same structure in the Z-axis direction as the others.
A voltage V0 is applied across terminals 126a and 126b, and a current I flows through electrode 123 to cause variations in voltage in the X-axis direction. Thereby, in addition to the cyclic changes in refractive index caused by the cyclic waveguide array, a linear refractive index gradient Δn is caused by these voltage changes as illustrated in FIG. 25. When light is emitted through one end of the waveguide, propagation of the light moves toward a high refractive index side. These oscillations in the X-axis direction are referred to as “optical Bloch oscillations”.
When voltage V0 changes, refractive index gradient Δn changes so that the amplitude of the optical Bloch oscillations changes. Thereby, switching of the output waveguide can be performed; Thus, applied voltage V0 can control a position of an output port.
In the variable demultiplexer disclosed in the foregoing “PHYSICAL REVIEW LETTERS”, as shown in FIG. 26, a waveguide array is formed by waveguides 132 arranged parallel to each other on a substrate 131. In the waveguide array, widths of waveguides 132 gradually increase as the position moves in the forward direction along the X-axis. Also, spaces between neighboring waveguides 132 decrease as the position moves in the forward direction along the X-axis. Each waveguide 132 has a uniform structure in the Z-axis direction.
In the variable demultiplexer shown in FIG. 26, changes in space between waveguides 132 and changes in width of waveguides 132 of the waveguide array cause the cyclic changes in refractive index as well as the refractive index gradient caused by the changes in effective refractive index. Consequently, when light is emitted into waveguide 132 through its one end, optical Bloch oscillations occur. When the power of the incident light increases, a nonlinear effect occurs, and the incident light does not follow the linear refractive index gradient. Consequently, the amplitude of the optical Bloch oscillations changes so that switching of an output waveguide can be performed. Thus, the output port can be controlled by changing the power of the incident light.
Such a device is also known that a photonic crystal is used as a principle of the demultiplexer, instead of the foregoing optical Bloch oscillations. The photonic crystal is an artificial structure in which two or more kinds of materials having different refractive indexes are arranged with a periodicity nearly equal to a wavelength of light. In the photonic crystal, a band structure with respect to an energy of photons is formed similarly to a phenomenon in which a periodic potential distribution in a solid crystal forms a band structure with respect to an energy of electrons. The photonic crystal has three features, i.e., PBG (Photonic Band Gap), anisotropy and dispersibility which are important for application.
Variable demultiplexers using the photonic crystals are disclosed, e.g., in Japanese Patent Laying-Open Nos. 2003-255160 and 2003-43277. Japanese Patent Laying-Open No. 2003-255160 has disclosed an optical wavelength multiplexer/demultiplexer employing a combination of AWG (Arrayed Waveguide Grating) and the photonic crystal. More specifically, according to Japanese Patent Laying-Open No. 2003-255160, a photonic crystal wavelength select filter of a grid modulation type is inserted into a groove extending perpendicularly to a slab waveguide on the output side of the AWG so that cost reduction can be achieved, crosstalk can be reduced and losses in output light provided from respective channels can be uniform. In this Japanese Patent Laying-Open No. 2003-255160, the AWG is utilized for the demultiplexing function, and a filter formed of the photonic crystal is added for achieving the cost reduction and low crosstalk.
Japanese Patent Laying-Open No. 2003-43277 has disclosed a wavelength demultiplexer circuit using a super-prism phenomenon. The super-prism phenomenon is caused by dispersibility of the photonic crystal, and a slight difference in wavelength within the photonic crystal causes large changes in propagation angle according to this phenomenon. The technique disclosed in this Japanese Patent Laying-Open No. 2003-43277 has a feature that the propagation angle in the crystal is significantly changed by the use of this super-prism phenomenon, and thereby the transmission path is demarcated according to the wavelength. Consequently, in this Japanese Patent Laying-Open No. 2003-43277, it is not necessary to form individual waveguides in contrast to the AWG, and fast operations, high packing densities, improved transmission efficiency and others can be achieved.
However, the prior arts already described suffer from the following problems.
In connection with the techniques disclosed in “OPTICS LETTERS” and “PHYSICAL REVIEW LETTERS”, the light propagates a long distance (which will be referred to as a “one-period propagation distance” hereinafter) of 10 mm or more in the Z-axis direction while a oscillation for one period occurs in the light traveling which oscillates in the X-axis direction due to the optical Bloch oscillations. Therefore, when producing the variable demultiplexers, problems of low yield and bad productivity are liable to occur. In particular, when producing the variable demultiplexers by Electron Beam (EB) exposure, it is impossible to produce each of chips in one field of the EB exposure step, and a field boundary is formed inside a structure of the variable demultiplexer. This may cause unstable features in the variable demultiplexer.
The techniques disclosed in Japanese Patent Laying-Open Nos. 2003-255160 and 2003-43277 do not have a function of switching an optical path in an optical transmission path, and these references have neither disclosed nor suggested a variable mechanism. In connection with the technique disclosed in Japanese Patent Laying-Open No. 2003-255160, when an AWG is once formed on a substrate, it is no longer possible to change the form of the slab waveguide on the output side and the structure of the array waveguides. Thus, the output wavelength in each output waveguide is fixed, and usually cannot be changed. Likewise, according to the technique disclosed in Japanese Patent Laying-Open No. 2003-43277, when the photonic crystal is once produced, it is no longer possible to change the structure such as a form and a size of a hole and a grating constant of the photonic crystal. Thus, each wavelength is output at a fixed position, and flexible switching is impossible.